Turning flowers into charcoal
It might be counterintuitive, but turning water flowers into hydrochar might be one of the most beneficial uses of hydrothermal carbonization. And one of the simplest.
We realize that putting waste biomass into an industrial pressure cooker and heating it until it turns into some brown coal-like mass is not the sexiest of topics, but today we’re upping the ante and using some really sexy biomass as input: hyacinth, lotus, and waterlily.
As attractive as they might seem to the casual observer or the infatuated lover, many water flowers belong to an invasive species and can turn into quite a pest for anyone whose subsistence depends on having an uncovered expanse of water.
Water hyacinths now cover over ten thousand hectares of Lake Victoria in East Africa, endangering the livelihoods of millions along its shores in Kenya, Tanzania, and Uganda. Similar problems arise in many other countries where invasive species have become a major economical and ecological problem.
This has opened an entrepreneurial opportunity to harvest the hyacinths (no worries about fresh supplies, they grow like gangbusters) and turn them into biofuel to replace the ubiquitous firewood, solving two problems at a time.
But we can add one step and solve a whole range of problems in addition to these two.
The cleansing power of water hyacinths, in two steps
One of the attractive features of water hyacinths is that it cleans the water, and so does char. We can combine the two and create an extremely simple, widely applicable two-step sewage plant.
The first stage is the hyacinth basin and, if necessary, a reed basin. The second step is the hydrochar basin. Combined they might not quite reach the quality of hi-tech sewage plants in industrial countries, but they do a lot of work in regions where clean water is hard to come by, they do it with a ubiquitous waste resource, and they do it in a low-tech setup suitable for emerging economies.
All that is needed is an industrial pressure cooker: an autoclave for hydrothermal carbonization, solar heat collected with a simple vacuum solar collector, and loads of water hyacinths. These can come from lakes or from the first basins. The hyacinths extract nutrients from the wastewater. These nutrients are then concentrated in the HTC process water and can later be used for fertilization.
The water hyacinths are now converted to hydrochar via hydrothermal carbonization. The hydrochar can be separated and serves as an absorbent for pollutants in the water pre-cleaned by the water hyacinths.
When the hydrochar is saturated with pollutants, it can still be dried in the sun and used as fuel. Or it could be landfilled, sequestering carbon. This could be sold via certificates to companies in industrialized countries to offset their emissions. A potential new export commodity for countries that have to import most of it, and a way out of debt?
The nice thing is that the nutrients collected by the water hyacinths are available again in the process water after hydrothermal carbonization.
For this purpose, it is necessary to break down the organic residues in the process water — for example by aeration — and then you get a solution containing nitrate/ammonium, phosphate, potassium. These solutions can then be used as liquid fertilizer and replace expensive imported fertilizers.
Another contribution to independence — and not to ignore: up to now, the water hyacinths have often only been removed from the water and deposited at the lakeshore. This is where they are left to rot. The nutrients are released and return to the lake (and the carbon released back into the air as CO2). The lake becomes eutrophic again, which leads to problems with more water hyacinths and algae that form further biomass in the lake with the released nutrients.
So adding this one process step improves lake water quality and lake biodiversity in parallel. And it creates a stable form of carbon which we can put to all good uses.